Heretofore, a demand has been made for coating resins of a one-pack type which could be cured sufficiently at low temperature (at 80.degree. C. or lower). To this end, some techniques have been proposed as described below, but none of them can provide a good balance of storage stability, curing performance and other performance characteristics required for a variety of coating compositions.
More specifically, with regard to a conventional curing system in which an acrylic polyol configured to have a hydroxyl group at its molecular side chains is cured with a melamine resin or a blocked isocyanate compound, or in which an epoxy resin is cured with the use of a potential catalyst (a curing agent), curing should be performed at 120.degree. C. to make the system effective. Lower curing temperatures than 120.degree. C. fail to give sufficient crosslink density with consequent unfeasibility of desirable film properties and the like. Curing at a lower temperature than the above counterpart needs multiple packs, leaving a number of problems at work. Polyisocyanate is satisfactorily curable, but is rather poisonous due to the nature of isocyanate.
Relatively recently, curing systems have been evolved and disclosed in which a metal chelate compound such as aluminum chelate compounds is added to an acrylic resin structured to have at their side chains an alkoxy silane group and an epoxy group (Japanese Examined Patent Publications Nos. 55-41712, 60-50223, 60-50225, 61-23816 and 61-23817 and Japanese Unexamined Patent Publications Nos. 64-75502, 4-139281, 1-259071 and 1-287177). However, because the acrylic resin has an alkoxy silane group (or a hydrolyzed silanol group) attached to the side chains, such a curing system brings about something unnatural in configuration and hence results in poor storage stability. In industrial sectors (typified as in-situ coating at construction sites), the curing system is used as a two-pack type when cured at a low temperatures of 80.degree. C. or below. In the case of use as a one-pack type, the curing system is not sufficiently curable at a low temperature of 80.degree. C. or below.
Moreover, since the acrylic resin is structured to have an alkoxy silane group at the side chains, the foregoing curing system poses the problem that it tends to cause seeding (cohesion and the like) on contact with a pigment, and the resultant film tends to dry and cure preferentially only on its surface upon completion of the coating (consequently, a solvent remains in the film longer and hence makes desirable film properties unattainable). Still another problem is that when the curing system is coated at high temperature or in a low temperature and dry atmosphere, the film becomes shrunk on the surface.
In addition, in Japanese Unexamined Patent Publication No. 62-119276, there is disclosed a primer composition constituted of an acrylic resin structured to have attached to its side chains an epoxy group and a hydroxyl group, a bisphenol A epoxy resin, a silane compound structured to have attached to its side chains an amino group or a glycidyl group, and an alkyl chelate compound. Japanese Unexamined Patent Publication No. 2-73825 discloses a resin composition containing (1) a hydroxyl group-bonded acrylic resin, an alicyclic epoxy resin, a silane compound and a metal chelate compound such as an aluminum chelate compound or the like, or (2) an (alicyclic) epoxy resin, a hydroxyl group-bonded acrylic resin, a silane compound and a metal chelate compound such as an aluminum chelate compound or the like, and a method of curing such resin composition. However, the last-mentioned publication does not in any way disclose that both the acrylic resin and the epoxy resin should contain an alicyclic epoxy group, nor does it disclose using a glycidyl group-containing silane compound together with both of the resins. Japanese Unexamined Patent Publication No. 10-87942 discloses a one-pack composition containing an acrylic resin having at its side chains a hydroxyl group and an epoxy group, a variety of silane compounds, a curing catalyst such as an aluminum chelate compound or the like, and an organic solvent. In this technique, however, the composition fails to cure sufficiently at a low temperature of 80.degree. C. or below and besides suffers from film shrinkage to an extreme extent. Thus, the technique leaves much to be desired in order to obtain rust proofness, bonding performance, other coating performance characteristics and film appearance with a good balance and at a high level.